1. Field of the Invention
The present invention relates to a flat type image display device that displays images by exciting fluorescent members to emit light.
2. Description of the Related Art
In recent years, with the increase of the screen sizes of image display devices, light and thin “flat panel displays” are gaining popularity in place of deep and heavy Braun tubes (cathode ray tubes; hereinafter “CRTs”). Examples of the flat panel displays of which research and development are being actively pursued, include liquid crystal displays (hereinafter “LCDs”) and plasma display panel (hereinafter “PDPs”). However, the images provided by conventional LCDs are dark and may be viewed head on or at only very small angles. The images provided by PDPs have low contrast. Accordingly, the demand is growing for a flat panel display that is as bright, as high in contrast, and as large in view angle, as the conventional CRT, and that can meet the request for the increasing of the screen size and the realization of high definition.
To respond the above-described demand, self-luminous flat panel displays, which cause fluorescent members to emit light using electronic rays as in the case of the conventional CRT, are also under development. In one such system, surface-conduction electron emitters (hereinafter “SCEs”), each of which is one kind of cold cathode, are arranged as a matrix on a glass substrate instead of the conventional hot cathode used in the conventional CRT, has been proposed by the same assignee as the present application, in Japanese Patent Laid-Open Nos. 64-031332, 7-326311 and the like.
FIG. 2 is a cut-away-perspective view schematically showing a constructional example of a flat panel display using these SCEs, namely, a surface-conduction electron emitter display (hereinafter “SED”). In FIG. 2, a front substrate (or a “face plate”) 10 for displaying images comprises fluorescent members 12, a metal back 13, and a high-voltage terminal 3 each formed on a glass substrate 11. On the other hand, a rear substrate (or a “rear plate”) 20 comprises two-dimensionally arranged SCEs 22 each formed on a glass substrate 21, and X wiring 23 and Y wiring 24 for driving the SCE 22. The front substrate 10 and the rear substrate 20 face each other, and are separated a predetermined distance therebetween by sandwiching support frames 2. Then these two plates are sealed up, and air is removed, thereby forming a vacuum case 1. On the rear surface side of the vacuum case 1, there is provided a drive circuit 4 including an IC group for driving the two-dimensionally arranged SCEs. The vacuum case 1 and the drive circuit 4 together constitute the above-mentioned SED.
An electron emitted from an SCE 22 used as a cold cathode is accelerated toward the metal back 13 used as an anode, and excites the adjacent fluorescent members 12 to emit light. To excite the fluorescent members 12 in this way, it is necessary to accelerate the electron used for excitation up to approximately 10 keV or higher. Currently, to increase the brightness of an image, one could increase the acceleration voltage or the current of the electron beam. However, when either the acceleration voltage or the current of the electron beam is increased, the amount of X-ray radiation generated by the impact of the accelerated electron against the fluorescent members also increases. In addition, because the surface of the vacuum case 1 is made of a transparent component member such as a glass substrate 11, or the like to allow for the output of the light beams emitted by the fluorescent members 12, the quantity of X-rays that cannot be absorbed by the transparent component member are radiated to the outside as leakage X-rays.
The International Commission on Radiological Protection (ICRP) states in its recommendations that any apparatus that includes a portion where electrons are accelerated by an energy not lower than 5 kV should be considered as a potential ionizing radiation source, and that any television set that is used at a home or at a place that the public can approach, should not make a leakage excessing 0.5 mR/h at any easily-approachable point 5 cm distant from the surface of the set when the set is operating in a normal operating condition. Accordingly, in order to meet this criterion, X-rays should be blocked on the front side of the face plate in the vacuum case 1, as a matter of course. In addition, because it is desirable to sufficiently block X-rays leaked from the rear plate 20 side in the vacuum case 1, when utilizing an SED as a television set or a display set, a display drive circuit comprising integrated circuits (hereinafter “ICs”) is often placed inside an enclosure at the back of the rear plate 20 in the vacuum case 1 (i.e., on the opposite side of the face plate). Meanwhile, Japanese Patent Laid-Open No. 3-165421 discloses the arrangement that uses Ba, Zr, Sr, or Pb as an X-ray absorbing film in a display tube such as a CRT. Also, Japanese Patent Laid-Open No. 3-261026 discloses the arrangement where copper is disposed on the back surface of the rear plate as a heat radiation member.